The disclosure is directed to a carbonylation process for producing acetic acid which includes separating a vapor product stream from a carbonylation reactor to produce a crude acid product comprising acetic acid comprising lithium cations and contacting the crude acetic acid product with a cationic exchanger in the acid form within a first treatment device to produce an intermediate acid product; and contacting the intermediate acetic acid product with a metal-exchanged ion exchange resin having acid cation exchange sites within a second treatment device to produce a purified acetic acid. Embodiments directed to operation of a treatment device comprising a plurality of sampling ports are also disclosed.